Breast-conserving therapy is a standard local treatment for breast cancer up to stage II after breast-conserving surgery. In this treatment, the occurrence of contralateral breast cancer has been reported as a late adverse event. One of the possible reasons is the effect of low-dose exposure to contralateral breasts by scattered radiation. While treatment planning is carefully conducted, a detailed evaluation of the dose distribution has not been performed yet. In this study, we evaluated the effect of scattered radiation on the contralateral breast by the difference in irradiation conditions on the affected breast using Monte Carlo simulation code PHITS. In the calculation, a model for the breast was created using the ICRP110 phantom. For the irradiation condition, 4-6 MV X-rays were used and the dose distribution of scattered radiation was evaluated. The contralateral dose was less than the target dose farther away from the inner edge of the affected breast. In addition, the larger the breast size, the effect of scattered radiation increased.